Parasitic draw, most likely. Do you have a multimeter with an ampere setting? If you don't, you're going to need to obtain one.
To do this, set up the meter to read how many amps are flowing. Read your specific instruction manual on how to do this. Once it's configured, disconnect the negative side of the battery and join the meter in series, so that current flows through the meter to get back to the battery.
Ideally you should see zero. Anything less than 30 milliamps (mA) is what I would consider normal. If you see anything significantly above 30 milliamps, for example 70 milliamps or greater then we'll diagnose further.
If you have a load tester (25 dollars at either harbor freight or menards can get you a good one), then go ahead and use that. If not, places like autozone will check your alternator for free in most cases if you don't want to buy the equipment.